Liquid–liquid equilibrium data for binary perfluoroalkane (C6 and C8) + n-alkane systems
Introduction
Perfluoroalkanes have attracted a great deal of interest because their properties mean that they have applications in various fields, for example in fluorous phase organic synthesis using fluorous catalysis (fluorous biphasic chemistry) [1], [2], [3], [4], [5], biomedicine [6], where they are used as oxygen carriers in blood substitutes [7], [8], [9]. The structures and properties of perfluoroalkanes differ from those of their corresponding hydrocarbons. Perfluoroalkanes have very strong covalent bonding, very low polarizability, and very weak van der Waals interactions [10], [11]. Consequently, they have low surface tensions, high densities, low dielectric constants, low refractive indexes, high vapor pressures, and high gas solubilities compared with their corresponding alkanes [11], [12]. A knowledge and understanding of the properties of perfluoroalkanes, both in their pure form and in mixtures, are of practical as well as theoretical significance.
In the last 60 years, experimental LLE data from numerous studies of perfluoroalkane + n-alkane mixtures have been reported [13], [14], [15], [16], [17], [18], [19], [20], [21]. In the early 1950s, such mixtures were believed to be completely miscible, in all proportions, and it was thought that standard solution theory would describe their LLE behaviors. However, examination of the reported experimental LLE data has shown that these mixtures display anomalous behavior, such as large positive deviations from ideality, and liquid–liquid immiscibility [11], [20].
We investigated the LLE data for three binary mixtures of perfluorohexane + n-hexane, perfluorohexane + n-octane, and perfluorooctane + n-octane as perfluoroalkane + n-alkane systems. In perfluorohexane + n-hexane system, several literature values up to upper critical solution temperature (UCST) have been reported [13], [14], [15], [16], [17], [18]. However, discrepancies among the literature values are recognized, especially in the vicinity of the UCST. On the other hand, in perfluorohexane + n-octane and perfluorooctane + n-octane systems, detailed measurements of the LLE data from lower temperatures up to UCST have not been performed, although a few experimental values have been reported [11], [13], [20], [21].
The object of our study is to make detailed and accurate determination of the cloud point data of three perfluoroalkane + n-alkane binary systems from lower temperatures to the UCST using a laser-scattering technique developed in our laboratory [22], [23], [24], [25], [26]. In addition, the experimental LLE data were represented by NRTL equations [27]. Finally, the activity coefficients obtained from LLE data were compared with those obtained from vapor–liquid equilibrium (VLE) data.
Section snippets
Experimental apparatus and procedures
The LLEs of binary mixtures containing perfluoroalkanes were determined by a cloud point method with a laser-scattering technique. Details of the apparatus and its operation have been described elsewhere [22], [23], [24], [25], [26]. Temperature control with a cooling medium was used to determine the cloud points over the temperature range of the LLEs studied. Temperatures were measured with a platinum resistance thermometer. The accuracy was estimated to be ±0.01 K. The uncertainty of the cloud
Results and discussion
The experimentally determined cloud points for the three binary mixtures – perfluorohexane (1) + n-hexane (2) or + n-octane (2), and perfluorooctane (1) + n-octane (2) – are listed in Table 2, Table 3, Table 4, and a comparison of these experimental data with the literature values [13], [14], [15], [16], [17], [18], [19] is presented in Fig. 1, Fig. 2. The UCSTs determined in this study are listed in Table 5 with those by several authors [11], [14], [18], [19]. These UCSTs were interpolated from the
Data reduction
Experimental cloud point data were represented by the NRTL equation [27]. The following constrains for the LLE were employed in the representation of the experimental cloud point data:The activity coefficients γi in Eq. (1) were expressed by the NRTL equation. The NRTL equation is shown in Eqs. (2), (3), (4):A constant value of 0.2 was adopted for the non-randomness parameter α12 in Eq. (3). Temperature
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